1. Field of the Invention
The present invention relates to an image processing apparatus and an image processing method which process multivalue image data associated with an area of interest on a print medium to print an image on the area by moving a printing unit relative to the area two or more times or moving a plurality of printing element groups relative to the same area.
2. Description of the Related Art
As a technology to alleviate density unevenness and stripes in an image printed by an inkjet printing apparatus, there has been known a multipass printing method which completes an image of an area on a print medium by performing a plurality of printing scans over that area. Even with the multipass printing method, however, there may occur a shift in a dot printed position between a preceding printing scan and a subsequent printing scan. Such a dot position shift or deviation can lead to variations in dot coverage rate, resulting in image impairments such as density variations and density unevenness.
To alleviate such image impairments, a method has been known which involves dividing image data in the form of multivalue image data before being binarized into different printing scans and then binarizing the divided multivalue image data independently of each other or in an uncorrelated manner (Japanese Patent Laid-Open No. 2000-103088). FIG. 9A shows an arrangement of dots printed according to image data processed by the method of Japanese Patent Laid-Open No. 2000-103088. In the figure, solid black circles 1501 represent dots printed in a first printing scan; blank circles 1502 represent dots printed in a second printing scan; and gray circles 1503 represent dots printed overlappingly by the first and the second printing scan.
With this arrangement, if a group of dots printed in the first printing scan and a group of dots printed in the second printing scan are shifted in the main scan direction or subscan direction, the dot coverage rate over the print medium does not change so much. The reason for this is that while areas newly emerge in which dots printed in the first scan and dots printed in the second scan overlap each other, there are also areas where two dots that are supposed to overlap each other do not overlap.
However, positively trying to make dots overlap by using the method of Japanese Patent Laid-Open No. 2000-103088 can deteriorate graininess and insufficient density. For example, in highlighted areas where graininess shows easily, it is preferred that a small number of dots (1701, 1702) be evenly scattered, a predetermined distance apart from each other, as shown in FIG. 9B. However, in the construction disclosed in the Japanese Patent Laid-Open No. 2000-103088, there occur in places areas in which dots are printed overlappingly (1603) or adjoiningly (1601, 1602), as shown in FIG. 9C. These dot lumps show and degrade the graininess. In high-density areas where importance is given to maximum density value, too many overlapping dots can expose blank areas, resulting in an insufficient density. So, the percentage of areas where dots overlap (dot overlap rate) is preferably adjusted not to cause undue density unevenness, graininess and insufficient density in output images.
Particularly when images are formed using a plurality of ink dots of the same color but of different sizes, as has been done in recent years, the levels of density unevenness, graininess and density insufficiency vary depending on what percentage of each size of dots is used in which grayscale region. That is, which of the density unevenness suppression, graininess reduction and the avoidance of density insufficiency should be given how much priority changes from one dot size to another. So, when a plurality of dot sizes are used, it is desired that a dot overlap rate be properly adjusted according to the dot size.
In the method disclosed in Japanese Patent Laid-Open No. 2000-103088, because a quantization operation is performed independently in individual planes without having these planes correlated to each other at all, the dot overlap rate is determined solely by the quantization method. That is, changing the dot overlap rate according to the dot size or adjusting the dot overlap rate within a target range have not been possible. As a result, it has been difficult to produce an image that is free from problematical levels of density unevenness, graininess and density insufficiency in the entire grayscale range.